Synthesis of alkylated boron compounds



United States Patent SYNTHESIS OF ALKYLATED BORON COMPOUNDS Earl L. Muetterties, Chadds Ford, Pa, assignor, to E. I.

du Pont de N emour's and Company, Wilmington, Del.,

a corporation of Delaware No Drawing. Application August 1, 1956 Serial No. 601,377

9 Claims. (Cl. 260-462) dialkylborinates, and the dialkyl alkaneboronates. These compounds may also be called trialkylboranes, alkoxydialkylboranes, and dialkoxyalkylboranes, respectively, but for consistency with presently-accepted nomenclature the former names will be used herein.

The above-mentioned compounds have generally "been made heretofore by multi-step reactions involving highly inflammable reaction media, spontaneously inflammable reactants, or both. The trialkylborines have, for instance, been prepared from boron halides by Grignard reactions in which highly inflammable ethers, e. g., diethyl ether, are normally used as solvents. Theborines have also been prepared from boron halides or borate esters by alkylation with the spontaneously inflammable dialkylzinc compounds themselves synthesized only with difliculty. They have also been prepared by interaction of olefins or hydrocarbons with the expensive and spontaneously inflammable diborane.

Recently (U. S. Patent 2,446,008) an improved synthesis for the trialkyl-borines has been found in the high temperature, vapor phase (at least 250 C.) reaction be tween a-boron halide and an alkyl halide in the presence of a halide acceptor such as metallic zinc, or aluminum. While this route ofiers obvious advantages over the prior art methods, the yields and conversions are necessarily low because of the well-known instability of the alkylboron compounds at the required elevated temperatures.

A primary object of this invention is, accordingly, provision of a novel and useful method for preparing alkyl borons of the types known as trialkylborines, R'- B, alkyl dialkylborinates, R' B (OR), and dialkyl alkaneboronates, R'B(OR) j Another object is provision of a method for preparing the named classes of compounds without the use of spontaneously inflammable solvents or reactants.

A further object is provision of a method for synthesizing these compounds at temperatures lower than those generally employed heretofore.

The above-mentioned and yet other objects are achieved in accordance with this invention by a process involving direct interaction between metallic aluminum, an alkyl borate ester, and an alkyl halide at relatively low temperatures.

In the preferred embodiment of the invention, an alkyl borate ester and an alkyl halide, both of relatively low molecular weight, are reacted together in the presence of metallic aluminum at from room temperature to 200 C., and most desirably between 50 and 150 C., for not more than about 8 hours. Either atmospheric or superatmospheric pressure is employed, the latter being used With the shorter-chain more volatile alkyl halides and resultant alkylboron compounds, especially when the higher reaction temperatures are desired. Products can be separated by distillation. a

The alkyl borate esters and the alkyl halides used in the. process of this invention, and the alkyl boron compounds obtained therefrom, include both cycloaliphatic and aliphatic compounds, i. e., the carbons of the various radicals linked, directly and respectively, to ester oxygen,-

halogen, and boron must be aliphatic in character.

The following examples are submitted to illustrate the invention further but not to limit it. Quantities of materials are given in terms of parts by weight.

Example I A pressure-resistant reaction vessel, lined with a halogen-resistant commercially available alloy of nickel, iron, and molybdenum known to the trade as Hastelloy C, was charged with 30.0 parts of tris-n-butyl borate, 7.0 parts of aluminum, 0.5 part of iodine, and 0.5 part of aluminum chloride. The bomb was closed, evacuated, charged with 30.0 parts of gaseous methyl chloride, and

then heated at 150 C. for three hours under'autogenous pressure. lowed to cool to room temperature and the volatile contents were distilled in a stainless steel cylinder cooled inv denced by mass spectrometer data, a small amount of n-butyl dimethylborinate was also obtained.

Example 11 As described in'Example I, a pressure-resistant reaction vessel ,was charged with 21.0 parts of trimethyl borate, 40.0 parts of 8- to 20-mesh aluminum, 5.0 parts of aluminum powder, and 0.5 part of iodine. The reactor was closed, evacuated, charged with 41.0 parts of methyl chloride as before, and then heated under autogenous pressure for three hours at C. After cooling to room temperature, there was obtained by distillation into a stainless steel cylinder, cooled in a liquid nitrogen bath, 35.0. parts of volatile products which by mass spectrometry were shown to contain 8588% of methyl chloride, 10- 11% of trimethylborine, and l.82.0% of trimethyl borate. The amount of trimethylborine obtained correspondsto a conversion of about 30%.

In substantially identical preparations, varying only in that the reactions were carried out for two hours at C. four hours at 200 C. and three hours at 200 C., trimethylborine was obtained in conversions or, respectively, 21%, trace amounts, and 3%. In this last preparation analysis on the mass spectrometer showed the presence of small amounts of methyl dimethylborinate and dimethyl methaneboronate. A similar preparation carried out at 200 C. for four hours produced a good yield of volatile products which by mass spectrometer determinations were shown to be 84-85% methane. After this product was pumped at liquid nitrogen temperatures there was left as a residue and longer reaction times on the process.

Example III parts of 20-mesh aluminum. The exit end of the con- At the end of this period, the bomb was a1- reflux for four hours. There was thus obtained in. the

cooled trap 602 parts of low-boiling material which by mass spectrometer .analysis was shown to contain mostly methyl iodide and small amounts of trimethyl- Upon preciborine plus a trace of trimethylborate. sion distillation of the liquid product, there was obtained 16.0 parts (97% yield and conversion) of trimethylborine boiling at -20 C. at atmospheric pressure.

, Example IV I In'the same manner outlined in detail in-Example III, except that the solid Dry Ice/acetone cooled trap was omitted, l20.0 parts of ethyl iodide was added dropwise over a period of one and one-half, hours to 3.6.0 parts of tn'methyl borate and 30.0 parts of 8-" to 20-inesh aluminum. At the endof the addition. the reaction mixture was heated at'the reflux for one and one-half. hours. The reactor was then cooled to about '70 C.

andthe system evacuated. An evacuated stainless steel.

cylinder was then connected ,to the system and the reactor warmed slowly to about 50 C. The volatile reaction products distilled over-into the cylinder. There was thus obtained 24.0 parts of volatile reaction products which by mass spectrometer analysis were shown tocontain mainly triethylborine and ethyl iodide.

The process of the invention is generic to the preparation of the aliphatic, and cycloaliphatic replacement. derivatives of the aliphatic and cycloaliphatic borate borate ester. For the completely substituted products, -i.e., the trialkylborines, stoichiometrically three moles of the alkyl halides are required for each mole of the borate ester. For higher conversions an excess of the alkyl halide will generally be used. Insofar as the aluminum is concerned, two moles are required for every three moles of the borate ester and every three moles of the alkyl halide for the dialkyl alkaneboronates; four moles are required for every three moles of the borate ester and every six moles of the alkyl halide for the alkyl dialkylbo'rin'ates; and two moles are required for every mole of the borate ester and every three moles of the alkyl halide in the case of the trialkylborines.

The invention is also generic to the use of borate esters which are aliphatic in character, i. e., where the three ester oxygens are each linked singly and directly to boron and otherwise singly and directly to a carbon aliphatic in character, i. e., which is involved only in single linkages. The preferred borate esters are those wherein the three ester radicals, which can be alike or different, are wholly hydrocarbon and of no more than twelve carbons apiece. Especially preferred borate esters are the wholly hydrocarbon, saturated, aliphatic and cycloaliphatic triesters of no more than eight carbons in any one ester radical and generally of no more than a total of twelve carbons. Suitable spccific examples of these borate esters, in addition to those of the foregoing examples, include aliphatic borate esters, e. g., tris-n-propyl borate, tris-dodecyl borate, trisoctyl borate, tris-isobutyl borate, tris-isoamyl borate, and the like; cycle-aliphatic borate esters, e. g., tris-cyclohcxyl borate, andthe like; and the less preferred araliphatic esters, e. g., tris-benzyl borate, and the like. For obvious reasons of readier availability, the wholly aliphatic hydrocarbon esters are preferred.

The process employs halides wholly aliphatic in character, i. e. where'the carbon immediately linked to the halide is involvedonly in single linkages, wherein the the tris-aliphatic and cycloaliphatic borines, the aliphatic;

and cycloaliphatic bis-aliphatic and cycloaliphatic borinates, and the bis-aliphatic and cycloaliphatic'alkaneand cycloalkaneboronates. schematically the process can be illustrated by the following general equations:

In these equations, R andR', which can be alike or different, represent monovalent hydrocarbon radicals, aliphatic in character, i. e., including both aliphatic and cycloaliphatic radicals, free of non-aromatic unsaturation and of no more than about twelve carbonsapiece, and preferably of no more than about eight carbons apiece- The by-products of the reaction are indicated as the aluminum trihalides and the aluminum tris-alkoxides.

It will be understood that a mixture of these products tvto mole f. the al yl-h lid for. e ch mole of the halogen is of atomic number from 17 to 53,'i. e., chlorine, bromine, and iodine. The preferred alkyl halides are those of no more than twelve and especially no more than eight carbons which, other than the single halogen, are otherwise hydrocarbon free of aliphatic unsaturation, i. e., free of non-aromatic unsaturation. Suitable specific examples of these halides, in addition tothose given in the foregoing examples, include: aliphatic halides, e. g., ethyl chloride, octyl iodide, dodecyl bromide, and the like; cycloaliphatic halides, e. g., cyclohexyl bromide, and the like; and the less preferred araliphatic halides, e. g., benzyl bromide, and the like. The bromides and iodides generally give higher conversions in shorter time, but this advantage is achieved only at the higher cost inherent in the bromides and the iodides.

The trialkylborines, the dialkyl alkaneboronates and the alkyl (dialkyl)borinates obtained range from relatively high boiling liquids in the case of the long chain derivatives to normally gaseous products in the, case of the shorter chain derivatives, with the volatility increasing in the boiling point decreasing, both as the number of alkoxy groups are. replaced and as the. number of carbons in each replacing radical decreases. Thus, the highest boiling materials are the long chain diesters of the long chain alkaneboronates and the 'lowestboiling products,

will enerally be present the alkane hydrocarbons and smaller amounts of the alkene hydrocarbons corresponding to the alkyl compounds leing used. The amount of this alkane hydrocarbon present will generally increase with both increasing reaction temperature and reaction time. Since this material in the case of the methyl compounds is a non-condensible gas and in any case irrespective of carbon content is a much lower boiling material, i. e., appreciably more volatile, than the desired boroncontaining products, it can easily be removed, if present, by pumping the volatile materials obtained from the reaction mixture while the latter are cooled at liquid nitrogen temperatures. Since the desired alkylboron compounds are all condensible under these conditions, they remain in the liquid nitrogen-cooled zone.

Metallic aluminum of substantially any degree of purity commercially available and in substantially any form can be used in the process. Because of more rapid reaction and generally also higher conversion, it is desirable to use metallic aluminum with as high a surface per unit weight as possible consonant with maintenance of a substantially uniform distribution of the metal in the reaction zone. Commiuuted metal in the form of aluminum powder, flake, granules, or turnings of various sizes is commercially available and convenient for use in the process.

it is frequently found efiicacious to add minor amounts of catalysts to the reaction mixture. Suitable catalysts include the aluminum halides or the halogens themselves which presumably function through the in situ formation of the aluminum halide. Aluminum chloride and iodine are two especially suitable catalysts.

The reaction proceeds effectively in the absence of any added diluent although obviously any of the conventional inert organic liquid reaction media can be used if desired. Suitable diluents include the liquid hydrocarbons and halogenated hydrocarbons, including in both instances the aliphatic, aromatic, and cycloaliphatic species, e. g. carbon tetrachloride, the heptanes, cyclohexane, and the like. Frequently the process willbe run using an excess of either the borate ester or the alkyl halide as a reaction medium, depending respectively on whether the alkaneboronates and the dialkylborinates or the trialkylborines are the major desired product.

The reaction can be accomplished over a wide range of temperatures from room temperature, ca. 20 C., to about 200 C The process will usually be operated at from 50- 150 C.

Reaction times will vary from a few hours to normally no more than about 24 hours considering the process for batch operations, although obviously the process can be operated quite suitably on a continuous basis. For a given batch, reaction times will generally range from about two to about eight hours. The longer periods may be used with the longer chain, less reactive alkyl halides.

Superatmospheric pressure can be used and is found particularly efiicacious in the case of the low boiling alkyl halides and the corresponding low boiling and relatively volatile alkylborone products. The superatmospheric pressure is generally produced autogenously.

By employing the above-described methods, the process of this invention can be used to prepare many wholly alpihatic borines, borinates, and boronates. Thus, tris- (dodecyl) borine is obtained from dodecyl bromide and tri-isobutyl borate; tris (octyl)borine is obtained from octyi iodide and tris-octyl borate; tris(propyl)borine is obtained from propyl iodide and tris-isopropyl borate; tris(cyclohexyl)borine is obtained from cyclohexyl bromide and tris-methyl borate; tris(benzyl)borine is obtained from benzyl bromide and tris-isoamyl borate; ethyl (dicyclohexyl)borinate is obtained from ethyl chloride and tris-cyclohexyl borate; dioctyl ethaneboronate is obtained from ethyl chloride and tris-octyl borate; dibenzyl methaueboronate is obtained from methyl chloride and tris-benzyl borate; octyl (dimethyl)borinate is obtained from methyl chloride and tris-octyl borate, and the like.

The products of this new process are well-known compounds of established utility. The borines, i. e., the trisalitylboranes, are of especial interest as high energy fuels in view of the extremely high heats of oxidation they exhibit. The borinates, i. e., alkoxydialkylboranes or alkyl esters of the dialkyl boric acids, and the boronates, i. e., dialkoxyalkylboranes or dialkyl esters of the alkyl boric acids, form convenient sources for the respective free acids and anhydrides through simple hydrolysis and dehydration.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. The method of preparing an. :alkylated boron compound which comprises reacting an alkyl borate ester and an alkyl halide of the group consisting of alkyl chlorides, bromides, and iodides in the presence of metallic aluminum at a temperature of about 20-200 C., the alkyl groups of said alkyl borate ester and said alkl halide containing not more than 12 carbon atoms each.

2. The method of claim 1 in winch the temperature is about 50-450 C.

3. The method of claim 1 in which the alkyl radical of the alkyl halide is cyclo'aliphatic.

4. The method of claim 1 in which the aluminum is comminuted.

'5. The method of claim 1 employing additionally an aluminum halide as a catalyst.

6. The method of claim 1 employing additionally a halogen as a catalyst.

7. The method of forming a trialkylborine which comprises reacting a trialkyl borate ester containing not more than 12 carbon atoms in each alkoxy group with an alkyl halide of the group consisting of alkyl chlorides, bromides, and iodides of not more than 12 carbon atoms in the presence of metallic aluminum at 20-200" C.

8. The method of forming trimethylborine which comprises reacting a trialkyl borate ester containing not more than 12 carbon atoms in each alkoxy group with a methyl halide of the group consisting of methyl chloride, bromide, and iodide in the presence of metallic aluminum at a temperature of about 20200 C.

9. The method of forming triethylborine which comprises reacting a trialkyl borate ester containing not more than 12 carbon atoms in each alkoxy group with an ethyl halide of the group consisting of ethyl chloride, bromide, and iodide in the presence of metallic aluminum at a temperature of about 20200 C.

No references cited. 

1. THE METHOD OF PREPARING AN ALKYLATED BORON COMPOUND WHICH COMPRISES REACTING AN ALKYL BORATE ESTER AND AN ALKYL HALIDE OF THE GROUP CONSISTING OF ALKYL CHLORIDES, B ROMIDES, AND IODIDES IN THE PRESENCE OF METALLIC ALUMINUM AT A TEMPERATURE OF ABOUT 20-200*C., THE ALKYL GROUPS OF SAID ALKYL BORATE ESTER AND SAID ALKYL HALIDE CONTAINING NOT MORE THAN 12 CARBON ATOMS EACH. 